The rumours about Windows possibly being ported to ARM has left a lot of people bewildered; why would you port Windows NT when Windows CE 6.0 is a perfectly capable operating system? Putting all the pieces together, it's actually quite clear why you would want Windows NT on ARM: servers.

Point is without any presence on ARM - if ARM servers take off Microsoft is completely locked out of a potentially huge growth market. Do you really think they are willing to let that happen?

This was my take on it as well. New power-efficient ARM chips that could be used in a desktop or server role are on the horizon, and they will be available to market in a couple of years. Some OEMs are sure to take advantage of such chips and make a server which achieves a significantly better power-per-performance metric than x86 or x86_64 can, perhaps it will be more than an order of magnitude better.

Currently, the only viable server OS for such a machine would be Linux.

IMO, Microsoft cannot tolerate that. If Microsoft want to stay in the server business in a couple of years time when these machines come to market, Microsoft will HAVE to have a Windows server on ARM product ready to roll. If they are even a little tardy, Linux will take over this entire market segment.